This invention relates to controlling a synchronous motor driving a square-law torque load, particularly when operated at variable speeds by a frequency changer.
Technological progress in recent years has led to the use of many methods of variable-speed operation of synchronous motors, including those methods which make use of a forced commutation frequency changer and those which employ a natural commutation (load commutation) frequency changer. The range of application for such methods has also expanded year by year from small and medium motors to large motors. Although the types of loads driven by these synchronous motors are also numerous, they can be divided into two general types: constant torque loads where a roughly constant torque is required irrespective of the motor speed N, and square torque loads where the torque required varies in proportion to the square of the speed N such as with fans, blowers or pumps.
FIG. 1 is a graph of the characteristics of a conventional drive control system for a synchronous motor. The armature voltage Va of the synchronous motor and the load torque T are plotted on vertical axes, while the speed N of the synchronous motor is plotted on the horizontal axis. As the graph shows, it was usual, in conventional synchronous motors, for the Va (armature voltage)/N (rpm) ratio to be controlled to a constant value when the load torque exhibits squared characteristics.
The conventional drive control system for a synchronous motor driving a square-law torque load gives rise to the following problems:
(1) In general, with a square-law torque load, the power required is proportional to N.sup.3, and the system is therefore usually operated with the rpm of the synchronous motor set at less than 100% of the rated figure, with relatively long periods of operation at 50 to 80% of the rating. In this typical operating range, the power factor PF.alpha. at the input to the control circuit is low, and a large amount of reactive current is taken into the control circuit from the input AC source.
(2) Since the excitation input voltage EF.sub.1 of the magnetic field input source is at its maximum when the speed N of the synchronous motor is 0, the capacity of the field input source is required to be large and the power factor of the current drawn from this supply source is poor.